The pneumococcus ranks among the four leading causes of infectious death worldwide. This single species accounts for a large proportion of respiratory tract (pneumonia, otitis media) and invasive (sepsis, meningitis) bacterial diseases. The development of efficacious vaccines for children has been based upon protection against the few capsule serotypes that commonly cause disease. However, this strategy is being eroded by the selection for and replacement by non-vaccine serotypes. The initial step in the interaction of the pneumococcus with its human host is colonization of the nasopharynx. Experience with childhood vaccination has demonstrated the critical importance to public health of this step, since interrupting colonization decreases transmission to the unvaccinated adult population and greatly amplifies prevention of disease among the population. Since higher rates and duration of carriage enhance the risk of transmission and disease, we have studied how pneumococci persist and are eventually cleared from the host mucosal surface. We have utilized a murine model to show that the acute inflammatory response to carriage is ineffective at clearing colonizing organisms. The key to the success of the pneumococcus in colonization (and disease), therefore, is its ability to evade the initial inflammatory response it elicits. Our findings during the prior funding cycle identified pneumococcal genes required to evade killing by neutrophils. Our recent results reveal that an esterase (Pce) capable of cleaving choline-containing substrates including host platelet-activating factor (PAF), a paracrine neutrophil activator, enhances colonization. Aim#1 examines how pneumococci target PAF to suppress neutrophil-mediated clearance during colonization. After the neutrophil response peaks, eventual clearance follows a CCL2-dependent influx of monocytes/macrophages to the nasal lumen. Recent data show that for this process to be sustained over the multiple weeks needed to clear colonization requires cytosolic signaling and sensing of cytokines by the IL1 receptor. Aim#2 determines how inflammasome activation and IL-1-signaling by this extracellular pathogen contributes to the eventual clearance of colonization. The increased burden of disease that characterizes early childhood is associated with higher carriage rates in part because each episode lasts longer. Through our insight in to the dynamics of carriage, in Aim#3 we will explore why there is a delayed influx of mucosal macrophages that leads to prolonged colonization during infancy. Thus, our continuing project is focused on the fundamental importance of bacterial interactions with professional phagocytes; the evasion of neutrophils and clearance by macrophages. Since mucosal colonization is the first step for many infectious agents, the detailed mechanistic understanding of the biology of colonization from this project will have broad relevance to microbial pathogenesis research.